In an inkjet printer, a color spot printed in a pixel position on a medium may consist of a number of overlapping dots of the same color ink or different color inks. As one example, a four color ink printer printing any combination of cyan, magenta, yellow, and black dots for a pixel position with, at most, one dot per color for a single pixel position can produce 16 different colors for a single pixel position without halftoning. If multiple drops (e.g., four) of the same color ink can be used when creating a color spot, the possible color combinations without halftoning can be over 10,000.
The possible color spots which can be printed by a particular printer is sometimes referred to as a palette of colors. Typically, the number of RGB colors that can be generated by a computer and displayed on the computer's display screen is much more than the palette of colors available for a particular printer. Thus, there will typically be some error between the color spot printed for a pixel and the ideal RGB color generated by the computer for that pixel position. Using error diffusion halftoning techniques, the error between the actual color printed by the printer and the true tone value to be reproduced for that pixel position is dispersed to nearby pixel positions. The colors then printed in those nearby pixel positions will compensate for the tone errors in other nearby color spots so that the overall tone in an area on the medium closely matches the true RGB tone generated by the computer.
Various things affect the accuracy of the image reproduction on the medium and the speed of generating the reproduction. For example, different types of media (e.g., glossy paper vs. coarse paper) absorb ink differently and thus affect the shade and size of the color spot. The error diffusion calculations which take into account the media type may be so complex as to be a bottleneck to the printing speed, and the optimum error diffusion may be different for different media. Other factors also limit the ability of the printer to accurately reproduce the RGB color tones while printing at a high rate of speed.